Once implanted, active implantable medical devices are typically programmed remotely, external to the patient, by means of a control console called a “programmer.” The verification of the operating parameters of the implant or the transmission of information recorded by the implant is carried out by electromagnetic transmissions, called “telemetry” in the technique in question. This console or programmer is provided with a telemetry head that is placed over the site of the implant. The head, often called an “antenna”, includes an induction coil that acts as an antenna and collects the magnetic field coming from the implanted device.
One conventional configuration usually employed provides a single antenna placed in the body of the programmer head. The induced signal collected by that antenna is then amplified, conditioned and processed to extract therefrom information communicated from the implant. This configuration, however, presents the disadvantage of receiving at the same time as the useful signal all of the surrounding radio-electric disturbances (i.e., noise), which are all the more difficult to eliminate because the band-width of the system must be large.
To increase the signal-to-noise ratio, it is disclosed in EP-A-0 661 077 and its corresponding U.S. Pat. No. 5,674,265, commonly assigned herewith to ELA Medical, to use a plurality of collecting coils and to process the collected signals using a particular linear combination, making it primarily possible to preserve the useful component of the signal and eliminate the major part of the noise components coming from parasitic sources. The improvement of the signal-to-noise ratio is indeed essential if one wishes to increase the data transmission rate and/or volume from the implant towards the programmer, for example, if one wants to download large data stored in the implant such as Holter recordings taken of an endocardial ECG over a duration of several hours representing a volume of several megabits of data.
To further improve the performance of the programmer, it is disclosed in EP-A-0 797 317 and its counterpart U.S. Pat. No. 5,741,315, also commonly assigned herewith to ELA Medical, to use a particular geometry of collecting coils, making it possible to collect signals such that the noise component is greatly reduced at the collection stage, even before any signal processing by the electronic circuits. EP 797317 and U.S. Pat. No. 5,741,315 proposes to use two distinct coils on a common magnetic circuit such as a ferrite cup, namely a reception coil and a compensation coil. The reception coil, for example, is wound on the core of the ferrite cup, while the compensation coil is wound coaxially on the periphery of the cup. In this manner, the magnetic induction field lines of the useful component cross the receiving coil only once, whereas they cross the compensation coil twice in opposed directions. On the other hand, remote parasitic inductions cross the two coils in the same direction and almost identically, which easily makes it possible to isolate the interfering signals in order to remove them from received signal and thereby leave the useful signal.
Further, WO-A-01/05467 (assigned to Medtronic) proposes to use two distinct antennas for reception, but the antennas are positioned to be concentric and coplanar. The signals resulting from the two windings are withdrawn so as to eliminate the remote disturbances, which induce identical signals that are opposite in phase. The signal emitted by the implant at short distance induces voltages of opposite phases but of different amplitudes, which are thus not cancelled and make it possible to extract from the surrounding noise a useful signal whose secondary treatment can be simplified.
With these known configurations implementing a plurality of coils, the differentiation is excellent, but to the detriment of the practical range. Indeed, the distance between the antennas is limited by the dimensions of the programmer head. An implant located more than few centimeters from the antenna is already regarded as a remote disturbance; the attenuation of the signal according to the head-implant distance increases then much more quickly than the traditional law of attenuation according to the inverse of the square of the distance of separation.
To mitigate this disadvantage, one can envisage to lay out the two antennas side by side. With this configuration, however, a “dead zone” appears in a region located between the antennas, forming a “differential well” where no signal can be received. This region overlaps with the capture surfaces of the antennas and reduces the useful surface area of reception of the head. In addition, the rejection of the signals in the dead zone varies very quickly from a spatial point of view, so that a small lateral movement of the head compared to the implant can generate a significant amplitude variation, including even an inversion of the phase of the useful signal.